This invention pertains to methods for making magnetic disks comprising glass substrates. This invention also pertains to methods for making glass substrates. This invention also pertains to methods for processing sheets of glass into workpieces.
It is known in the art to manufacture magnetic disks (e.g. disk 1 of FIG. 1) by sputtering an underlayer 2 (e.g. Cr or a Cr alloy); a magnetic alloy 3 (e.g. Co, a Co alloy or a Fe alloy); and a protective overcoat 4 (e.g. carbon) on a disk-shaped glass substrate 5. Substrates used in the manufacture of magnetic disks must be extremely flat and smooth. Accordingly, a great deal of effort and expense is taken to polish glass during the manufacture of substrates.
A typical process for manufacturing glass substrates is as follows.
1. First, a sheet of glass 10 (FIG. 2) is formed using a float or draw method. Glass produced by these methods is typically very smooth.
2. Glass sheet 10 is cut into individual squares 12 (typically 100 mm by 100 mm, with a thickness of about 1 mm).
3. Thereafter, disk-shaped substrates 14 are cut out of squares 12. This requires cutting a centrally defined circular inner portion 16 and an outer portion 18 away from squares 12. This is accomplished using a sawing process or a scribing and breaking process.
4. Chamfers 22 are formed at the inner and outer diameter edges 24, 26 of substrate 14. (FIG. 3 illustrates substrate 14 in cross section, and shows chamfers 22.) Forming chamfers 22 requires multiple lapping and polishing steps.
5. Substrates 14 are then finished so that their surfaces are very smooth and have few or no defects. This is accomplished through one or more lapping or grinding steps and one or more polishing steps.
The grinding, lapping and polishing steps described above are extremely time consuming and expensive. During these steps, as much as 250 to 1000 xcexcm of glass are removed to ensure that the resulting substrate is extremely flat and smooth, e.g. having an Ra of about 2 to 4 xc3x85. (Typically, glass as formed by the float or draw methods is very smooth, but not very flat. Chamfering and various manufacturing steps described above introduce scratches and damage into the glass surface. Thus, polishing is performed to a) make the substrates flat, and b) cause the glass to once again be smooth.) To do this, the polishing steps add a great deal of manufacturing cost to the substrates. It would be desirable to find a way to eliminate the need for such polishing.
It is also known to use glass to manufacture LCD displays. An example of such a method is discussed in U.S. Pat. No. 5,622,540, issued to Stevens. Stevens provides a protective overcoat over a glass workpiece, and then scribes and breaks his workpiece as part of an LCD display manufacturing process.
Applicant is aware of another LCD glass manufacturing process (used by a company outside the United States) that comprises covering glass sheets with a protective layer, and then placing the glass sheets vertically in a holder that is used to transport them to the next work station. Interestingly, this process is used by a manufacturer that manufactures both glass magnetic disk substrates and glass for LCD displays. When manufacturing glass for magnetic disk substrates, the manufacturer did not apply a protective overcoat to glass sheets.
The reason for the difference in the way glass was handled during LCD display manufacturing and glass disk substrate manufacturing is that during disk substrate manufacturing, disks are subjected to extensive polishing and lapping. (This is particularly true because such lapping and polishing is necessitated by the chamfering steps.) Thus, there was no perceived reason for providing protective overcoats to glass used for disk substrate manufacturing, since the glass would be extensively lapped and polished anyway.
Applicant is also aware of a coring process wherein several square or rectangular sheets of glass are stacked and bonded together with an adhesive. The coring is done using saw like instruments to cut out the inner diameter opening and to cut out the disks from the squares. The adhesive is necessary so that all of the squares in the stack remain stationary during this operation.
In any event, it would be desirable to provide a glass substrate manufacturing process with reduced or minimized lapping to thereby reduce manufacturing costs.
It has been discovered that one of the reasons such extensive polishing is required during the manufacture of magnetic disks is that glass splinters lodge on the surface of the glass during the various manufacturing steps described above. These splinters scratch and damage the glass surface, thereby necessitating extensive lapping and polishing. By providing a protective layer over the glass during the various manufacturing steps, the need for such extensive processing is minimized.
In one embodiment, in lieu of chamfering the substrate edges, the substrate edges are rounded with an edge polishing technique. Whereas in the prior art, it was a foregone conclusion that the substrate would have to undergo extensive lapping and polishing (e.g. as a result of chamfering), it has been discovered that such lapping an polishing can be substantially avoided or minimized. Because of this, in a process in accordance with the present invention, it is now advantageous to protect a glass workpiece with a protective layer.
In accordance with one embodiment of the present invention, a protective layer is formed at the beginning of a glass manufacturing process, e.g. when the glass is in the form of a sheet. The protective layer is then removed after the glass is put into its final form (e.g. a glass substrate for disk manufacturing). Thus, in this embodiment, the protective layer stays on the glass through much or all of the substrate manufacturing process, and is removed at or near the end of the substrate manufacturing process.
In accordance with another embodiment of the present invention, the protective layer is placed on a glass workpiece before the workpiece is subjected to a process step during which the workpiece can be damaged. After the process step, the protective layer can be removed. This process step can be sawing, scribing, breaking, transporting the glass from one work station to another, edge polishing substrates, stacking substrates, chamfering substrates (e.g. using a laser or mechanical means), or other process step. Protective overcoats can be placed on the glass and removed from the glass at several times during the manufacturing process. In accordance to some embodiments of the invention, a protective overcoat can be place on a workpiece during single-workpiece processing or transportation.
One embodiment of a method in accordance with the invention begins with the step of providing a glass sheet. The glass sheet is typically formed by the float method. However, the glass sheet can be formed by other techniques such as drawing.
One and typically both sides of the glass sheet are then covered with a protective layer. The protective layer can be a polymer or other material, and protects the surface (or surfaces) of the glass sheet from splinters and other contaminants. Of importance, the protective layer prevents the surface of the glass sheet from being scratched by these particles.
After being covered with a protective layer, in one embodiment, the glass sheet is cut into smaller workpieces. This can be done by any of a number of methods, e.g., sawing or scribing and breaking. For the case of scribing and breaking, the scribing can be performed mechanically (e.g. using a diamond scribe tool). Alternatively, radiant energy (e.g. a laser) can be used to scribe the glass sheet. (Portions of the protective layer are typically burned away during laser scribing. During mechanical scribing or cutting, portions of the protective layer are typically cut. However, the protective layer remains on, and continues to protect, the portions of the glass workpieces that eventually serve as the major surfaces of the substrates being formed.)
In one embodiment, after laser scribing, the glass sheet is subjected to an acidic etching solution comprising fluoride ions. It has been discovered that this etching solution preferentially attacks the portions of the sheet that have been exposed to the laser. Because of this, the edges of the workpiece are smoother and better defined than they would be using other scribing and breaking techniques.
In one embodiment, both sides of the glass sheet are scribed simultaneously. This also improves the definition of the edges of the resulting smaller workpieces.
After the above-mentioned cutting or scribing and breaking process, individual substrates are cut out of the smaller workpieces. This can be accomplished using any of the techniques described above for cutting the glass sheet into smaller workpieces. Of importance, the protective layer remains on the workpieces during this process, and continues to protect the workpieces from being scratched.
Thereafter, chamfers are formed on the substrates. This can be performed in several different ways. For example, in one embodiment, the chamfers are formed using a laser cutting technique. In another embodiment, in lieu of forming chamfers, the comers of the substrate are rounded using an edge polishing technique. This process is performed by applying an edge polishing material (e.g. a slurry containing CeO2 or other edge polishing material) to the edge of the substrate. As explained below, this polishes the substrate edge and rounds the comers of the substrate.
In one embodiment, several substrates are stacked, and their edges are polished simultaneously. Of importance, the protective layer remains on both sides of each of the substrates, and continues to protect the substrates during this process. The substrates are then destacked.
It is at this point in the process that the protective layers are removed from the substrates. Of importance, because the protective layers have been on the substrates throughout the above-mentioned processing steps, the substrates are smooth and unscratched. Only minimal polishing (if any) is needed to put the substrates in condition for disk completion. The disks are typically completed by depositing, e.g. by sputtering, an underlayer, magnetic layer and protective overcoat, in that order, on the substrates. A lubricant layer is then applied to the protective overcoat.